We wish to study in the organ of Corti of the mouse the interrelations between the hair cell and the spiral ganglion fibers innervating it when the entire system has been exised from the organism and develops in vitro. Morphological observations will be made using light microscopy for the study of the live tissue; the light and electron microscopes will be used for studies of fixed material. The three projects of immediate interest are: 1) Development and maintenance of innervation in explants excised at different developmental stages when the hair cell region remains intact and when it is subjected to selective injuries; 2) Study of developing synaptic profiles; 3) Ultrastructural appearance of nerve endings: of fibers that grow by elongation in reaction to the displacement of their receptors; and of fibers that grow freely.